Latest 1.0.2
License MIT
Platforms ios 8.0


CI Status

New in 1.0.2

  • Added TypesafeCountType.
  • Added TypesafeIntegerIndexType.
  • Added support for returning TypesafeCountType instances when querying the size of an Array, Set, or Dictionary.
  • Added support for Array subscripting using TypesafeIntegerIndexType instances.
  • Added the ability for any UniqueIntegerType-conforming type to initialize itself with an Int and to retrieve the underlying value as an Int. The result for either operation is nil when the operation can’t be performed.

Stay tuned for upcoming arithmetic operators being added to UniqueIntegerType and UniqueFloatingPointType. There are a lot of those, so it will take a while but support for them is coming.

New in 1.0.1

UniqueBooleanType now supports all the standard boolean operators (!, &&, and ||).


I’m sure it has happened to you that you have properties in your code of the same primitive type but with vastly different semantic meanings. For example, suppose you have the following value types (admittedly, a bizarre example):

    struct User {

        let id: Int
        let name: String
        var isConnected: Bool
        var coreTemperature: Double
        var numberOfPets: Int

        init(id: Int, name: String, coreTemperature: Double, numberOfPets: Int = 0) {
   = id
   = name
            self.isConnected = false
            self.coreTemperature = coreTemperature
            self.numberOfPets = numberOfPets


    struct NuclearReactor {

        let id: Int
        let name: String
        var isConnected: Bool
        var coreTemperature: Double
        var numberOfFalseAlarms: Int

        init(id: Int, name: String, coreTemperature: Double) {
   = id
   = name
            self.isConnected = false
            self.coreTemperature = coreTemperature
            self.numberOfFalseAlarms = 0


Surely, the number of pets a user has and his or her user id have nothing to do with one another. However, the compiler will happily let you set the value of the user id to the count of his pets, as in

    let numberOfPets = 3
    let user = User(id: numberOfPets, name: "John Doe", coreTemperature: 36.7)

Perhaps you’ll argue that you wouldn’t possibly confuse the user id with his number of pets. Ok, then what about initializing a nuclear reactor using the user’s id rather than the reactor’s id, as in

    let userId = 10
    // ... sometime later ...
    let id = userId
    // ... sometime later, after you've forgotten that `id` is set to `userId` ...
    let reactorId = id
    let reactor = NuclearReactor(id: reactorId, name: "Homer's Favorite", coreTemperature: 10_000.0)

or initializing a user with the reactor’s core temperature, like so:

    let coreTemp = reactor.coreTemperature
    // ... sometime later ...
    let user2 = User(id: userId, name: "Jim Doe", coreTemperature: coreTemp)

These mistakes are far too easy to make and happen far too frequently. Wouldn’t it be great if the compiler could warn you when you’re setting the user’s core temperature with the reactor’s core temperature? Or, more generally, when you’re trying to set a value of one type to the value of another when both types use the same underlying primitive type for their storage?

Well, that’s what this library lets you do.

Addendum: You might enjoy reading this blog post by a colleague of mine. It goes into some more detail on why you should avoid what my friend calls primitive obsession. It’s a very good read.

How to use

There are four protocols,

  • UniqueBooleanType: for custom boolean types.
  • UniqueIntegerType: for custom types based off of any kind of integer.
  • UniqueFloatingPointType: for custom types based off of any kind of floating point type.
  • UniqueStringType: for custom string types.

and two structs that you need to know about:

  • TypesafeIntegerIndexType: to represent indices in arrays of a given element type.
  • TypesafeCountType: to represent the number or count of items of any given type.

To use the protocols, first declare types like the following somewhere in your project. You’ll need them but you won’t be using them directly all that often:

    struct Id<T>: UniqueIntegerType {
        public typealias PrimitiveType = Int
        public let value: Int
        public init(_ value: Int) {
            self.value = value

    struct Name<T>: UniqueStringType {
        public typealias PrimitiveType = String
        public let value: String
        public init(_ value: String) {
            self.value = value

    struct Connected<T>: UniqueBooleanType {
        public typealias PrimitiveType = Bool
        public let value: Bool
        public init(_ value: Bool) {
            self.value = value

    struct CoreTemperature<T>: UniqueFloatingPointType {
        public typealias PrimitiveType = Double
        public let value: Double
        public init(_ value: Double) {
            self.value = value

You could also create types that are not generic and types that aren’t structs. For instance, if in your application the only type that supports passwords is the User type and you want to make the password a class rather than a struct, then you could declare something like this:

    class Password: UniqueStringType {
        public typealias PrimitiveType = String
        public let value: String
        public init(_ value: String) {
            self.value = value

Regardless, the next thing to do is to create your actual data types, like so:

    typealias UserId = Id<User>
    typealias UserName = Name<User>
    typealias UserConnected = Connected<User>
    typealias UserCoreTemperature = CoreTemperature<User>
    typealias CountOfPets = TypesafeIntCount<Pets> // assuming you have declared a `Pet` type

    struct User {
        let id: UserId
        let name: UserName
        let password: Password
        var isConnected: UserConnected
        var coreTemperature: UserCoreTemperature
        var numberOfPets: CountOfPets

    typealias ReactorId = Id<NuclearReactor>
    typealias ReactorName = Name<NuclearReactor>
    typealias ReactorConnected = Connected<NuclearReactor>
    typealias ReactorCoreTemperature = CoreTemperature<NuclearReactor>
    typealias CountOfFalseAlarms = TypesafeIntCount<FalseAlarm> // assuming you have declared a `FalseAlarm` type

    struct NuclearReactor {
        let id: ReactorId
        let name: ReactorName
        var isConnected: ReactorConnected
        var coreTemperature: ReactorCoreTemperature
        var numberOfFalseAlarms: CountOfFalseAlarms

Note how nicely the various types read. For instance, Name<User> and Id<User> couldn’t be any clearer or more self-documenting, in addition to being type-safe.

Also note that you don’t have to declare a struct for counts. Rather, all you need to do is typealias a particular concrete version of the generic unique integer count type TypesafeIntCount. And if you don’t want your counts to be backed by Int instances but, instead, say, by UInt instances, then you can use TypesafeUIntCount instead. Even more generally, there’s a generic struct TypesafeCountType<CountType: Integer, TargetType> that you can use to declare unique count types with any kind of Integer backing.

Now, whenever you try to set values for one property using values from another, or pass values of the wrong type to a function, the compiler will warn you that your types are mismatched. For example, this code block won’t compile:

    let reactor = NuclearReactor(id: 100,
                                 name: "Homer's Favorite",
                                 isConnected: true,
                                 coreTemperature: 10_000.0)

    let coreTemp = reactor.coreTemperature

    let user2 = User(id: 5,
                     name: "Jim Doe",
                     isConnected: false,
                     coreTemperature: coreTemp)

because you’re trying to set the user’s core temperature to the reactor’s core temperature. Here’s a screen shot of the warning I get from Xcode in this case:

sample warning

But, wait! Have you noticed that I didn’t have to use the long form for calling the initializers of the new types we created, as in the code below?

    let reactor = NuclearReactor(id: ReactorId(100),
                                 name: ReactorName("Homer's Favorite"),
                                 isConnected: ReactorConnected(true),
                                 coreTemperature: ReactorCoreTemperature(10_000.0))

That’s because these types conform to protocols such as ExpressibleByIntegerLiteral, where appropriate, and that lets you use literal values rather than explicit initializer calls. Unfortunately, that’s only the case if these are literals. You may sometimes have to write things like

    let userName = UserName("John Doe [(userId.description)]")

because the compiler won’t accept

    let userName = "John Doe [(userId.description)]"

since the right-hand-side is no longer a literal string value.

Support for Arrays, Sets, and Dictionaries

But, wait! There’s more!

What if you have, say, an array of user ids? How do you process the primitive values hidden inside? Easy. You use the boxed() and unboxed() functions. For example,

    let array1 = [1, 2, 3]
    let userIds: [UserId] = array1.boxed()

will create an array of UserId instances from an array of the primitive type (in this case, Int). Similarly,

    let array2: [Int] = userIds.unboxed()

will give you back an array of the primitive values tucked inside. Note that you are required to help the compiler to infer the correct types by declaring the type of the variable you expect back, as was done above for userIds and array2.

This ease of going back-and-forth between the boxed and unboxed variants of collections of your custom types applies to instances of Array, Set, and Dictionary. For example, you could have a dictionary mapping user ids to user names, like this:

    var userIdToUserNameMap: [UserId, UserName] = [:]

Wait… don’t you have to worry about making the dictionary key type conform to Hashable? No, you don’t, because primitive types are already hashable and the underlying protocol that makes this library work uses that fact to implement Hashable on your behalf. As a result, all your custom WTUniquePrimitiveType creations aready conform to Hashable. They’re Equatable and Comparable too!

Array, Set, and Dictionary item counts

New in version 1.0.2 is the support for returning unique count types when you query the size of one of these kinds of collections. Say you have an Array of user ids and a Set of nuclear reactions, like so,

    let array: [UserId] = ...
    let reactors: Set<NuclearReactor> = ...

If you access the computed property count on these, you’ll get the standard Int values but if you want to get type-safe counts, you can access typesafeCount instead, as it returns a count that is typed specifically for the type of the elements stored in the collection at hand. So,

    let userIdCount = array.typesafeCount
    let reactorCount = reactors.typesafeCount

will have different types:

  • userIdCount is of type TypesafeIntIndexType<UserId>, and
  • reactorCount is of type TypesafeIntIndexType<NuclearReactor>.

More generally, the type of an array, set, or dictionary typesafeCount property is TypesafeIntIndexType<Element> where Element is the type of the instances stored in that collection.

For dictionaries, you can also get separate type-safe counts for the keys and the values, using typesafeKeyCount and typesafeValueCount. Of course, their underlying values are the same as what you’d get from typesafeCount and count.

Array indexing and subscripting

Also new in version 1.0.2 is the ability to index and subscript arrays using type-safe indices. So, now, you can have a type such as IndexOfUserId by declaring

    typealias IndexOfUserId = TypesafeIntIndexType<UserId>

and your array of UserId entries can be indexed and subscripted by this new type:

    var userIds: [UserId] = ...
    let userId = userIds[IndexOfUserId(2)] // index 2 returns the 3rd element, as usual
    userIds[IndexOfUserId(4)] = UserId(...) // sets a new userID for the 5th array entry

Having type-safe indices prevents the common mistake of passing indices of one type of array to another type of array.

Special support for Integer-backed unique primitive types

Sometimes you want to conserve memory by using an Integer property that isn’t an Int but something smaller, such as Int16 or Int8. Other times you want to guarantee that the integer in question is unsigned, so you want something like UInt. And sometimes you might want to have a property that is both at the same time, like UInt32.

In all of these cases, you often still want to be able to initialize them with Int values (when possible) and to refer to them as plain old Int instances (again, only when possible).

Well, now, in version 1.0.2, you can (try to) initialize any Integer-backed unique primitive type with Int values and you can (try to) access the underlying value as an Int, using the computed property valueAsInt (the value computed property still and always returns the underlying value, typed accordingly). Note that both the initializer and valueAsInt return nil when the conversion of the underlying Integer type to and from Int isn’t possible.

For example, you can’t initialize an Int8-backed unique primitive type with an Int value over 127 or any unique primitive type backed by an UnsignedInteger type with a negative Int. Similarly, a unique primitive type backed by UInt64 has a wider range of values than Int so you can’t always get an Int representation of the underlying value.

Arithmetic operations

That is something I haven’t done yet but plan to add to version 1.1 of this library, so stay tuned. For now, you’ll have to resort to access the internal values by using the getter property value, as in:

    let total = user.numberOfPets.value + 3

Soon you’ll be able to write

    let total = user.numberOfPets + 3

instead, and will then be warned if you write

    let total = user.numberOfPets + reactor.numberOfFalseAlarms

Well, you get warned now too but for a different reason, namely, that the operator + can’t be applied with operands of the given types.


The library enjoys 100% test coverage.

How it works

There is a base protocol

    public protocol WTUniquePrimitiveType: CustomStringConvertible, Equatable, Comparable, Hashable {

        associatedtype PrimitiveType: CustomStringConvertible, Equatable, Comparable, Hashable

        var value: PrimitiveType { get }
        init(_ value: PrimitiveType)


requiring any conforming types to declare storage of the appropriate type and an initializer for that storage.

Then, there are second-level protocols for the individual primitive types. For example, here’s the protocol for any primitive that’s based off of an Integer:

    public protocol UniqueIntegerType: WTUniquePrimitiveType, ExpressibleByIntegerLiteral {

        associatedtype PrimitiveType: Integer


The rest is just a matter of using protocol extensions to implement the common behavior that you expect these types to have.


This library was built with and for Swift 3.1 and Xcode 8.3.2.


WTUniquePrimitiveType is available through CocoaPods. To install
it, simply add the following line to your Podfile:

pod "WTUniquePrimitiveType"

If that doesn’t work (occasionally it doesn’t), try this instead:

pod 'WTUniquePrimitiveType', :git => ''


wltrup, [email protected]


WTUniquePrimitiveType is available under the MIT license. See the LICENSE file for more info.

Latest podspec

    "name": "WTUniquePrimitiveType",
    "version": "1.0.2",
    "summary": "Lets you create distinct types based on identical primitive types.",
    "description": "Say that your code has a user identifier that's an Int. The count of usersnis also an Int but, clearly, these are semantically different Ints. What ifnyou accidentally set the value of userId to the count of users? The compilernwouldn't complain because both values are Int instances. This library letsnyou create distinct types for situations like this, where you neednsemantically different types based on the same primitive type. It works withnall primitive types, not just Ints, and makes using these distinct types asneasy and transparent as possible.",
    "homepage": "",
    "license": {
        "type": "MIT",
        "file": "LICENSE"
    "authors": {
        "wltrup": "[email protected]"
    "source": {
        "git": "",
        "tag": "1.0.2"
    "platforms": {
        "ios": "8.0"
    "source_files": "WTUniquePrimitiveType/Classes/**/*",
    "pushed_with_swift_version": "3.0"

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